A cooling fan of an internal combustion engine blows cooling air onto a radiator that discharges heat from a cooling fluid of the internal combustion engine in order to cool the radiator. A fan coupling device that connects the cooling fan to an input shaft driven by the internal combustion engine transmits torque using a viscosity of a working fluid.
JP2007-321622A, published by the Japan Patent Office in 2007, proposes a malfunction diagnosis device for a fan coupling device that diagnoses a malfunction in the fan coupling device when a rotation speed of a cooling fan remains in a low state even though the fan coupling device is fully locked.